The present invention relates to a flow control valve, particularly to a flow control valve capable of accurately controlling the flow rate of a fluid in a wide range.
A method of controlling the flow rate of a fluid by a needle valve has been known to public. However, the size of the body of a needle valve for controlling the flow rate of a fluid in a range from 0 l/min. to 100 l/min. is so large relative to that of piping upstream and downstream of the valve that the valve is hardly put in practical use. Besides, there is no linearity between the degree of opening of the needle valve and the flow rate.
Under such circumstances, a pneumatic automatic control valve such as a diaphragm valve and a butterfly valve, an electric automatic control valve such as a ball valve, the degree of opening of which is regulated, another electric automatic control valve, which regulates the cross-sectional area of a fluid passage as disclosed in the Japan Patent Application (OPI) No. 121381/83 (the term "OPI" as used herein means an "unexamined published application"), and so forth have been put in practical use as flow control valves.
For example, a flow control valve shown in FIG. 3 has been put in practical use. In the valve, the cross-sectional area of the opening between a valve seat 36 and a valve head 33 constituted by a disk made of a resin and shaped as a truncated cone is regulated. However, since the valve head 33 is similar in constitution to an ordinary open/close valve, a fluid flows at a relatively high rate through the flow control valve even if the degree of opening thereof is very small. In other words, the valve cannot control the flow rate of the fluid to be low. This is a problem. Besides, since the flow rate of the fluid is controlled in terms of the clearance between the valve seat 36 and the valve head 33 made of the resin disk, the valve head is deformed due to the contact thereof with the valve seat 36 in the long-period use of the valve so as to change the clearance to alter the flow rate. This is also a problem.
To solve the problems, the present applicant proposed a flow control valve as disclosed in the Japan Patent Application (OPI) No. 35185/89. The valve is characterized in that a valve head is moved up and down by an electric motor whose rotation is converted into the straight movement of the valve head through a rack-pinion mechanism and a coupling plate; a spindle is attached to the coupling plate so that the spindle is vertically moved as the coupling plate is vertically moved; an inlet casing concentric to the spindle, and an outlet casing coupled with the inlet casing are provided; the valve head, which is cylindrically or conically shaped, is provided under an open/close contact surface tapered downward and shaped as a truncated cone, and can be put into the outlet casing; the form of the valve head is determined to set the cross-sectional area of the opening between the surface of the head and a valve seat so that the ratio of the change in the flow rate of a fluid to the lift of the valve is relatively small and the relationship between the flow rate and the valve lift is linear; and the valve head is located in the inlet casing when the valve is fully opened. The valve is shown in FIGS. 4 and 5. The valve head 46 is put into the outlet casing 47 to change the size of the opening between the head and the casing so as to control the flow rate of the fluid. The open/close contact surface 45 shaped as a truncated cone comes into contact with the valve seat 48 when the valve is completely closed.
The flow control valve disclosed in the Japan Patent Application (OPI) No. 35185/89 is high in the accuracy of the control of the flow rate in a wide range. However, since the outlet casing 47, the valve seat 48, the valve head 46 and the open/close contact surface 45 are made of metals, it is difficult to tightly fit the valve seat and the contact surface on each other. In other words, a small gap is likely to be made between the valve seat 48 and the contact surface 45 to deteriorate the accuracy of the control of a low flow rate by the valve and make the complete closure thereof impossible, to render it necessary to further provide a stop valve for complete shutting. This is a problem.
The present invention was made in order to solve the problem. Accordingly, it is an object of the present invention to provide a flow control valve which is higher in the accuracy of the control of the flow rate of a fluid in a wide range even if the flow rate is low and which can be completely closed so as to make it unnecessary to further provide a valve for complete shutting.